CEBALRAI (Beta Ophiuchi). At the northern tip of Ophiuchus, the Serpent Bearer, lies Rasalhague, the bright Alpha star.
Helping to form the huge rough pentagon that makes the classic
figure is Cebalrai, at the pentagon's northeastern corner, its
position and bright third magnitude status (2.77) causing Bayer to
call it "Beta." Unusual among star names, "Cebalrai" is pronounced
with a soft "C," even though it derives from a hard "K" sound in
the original Arabic. The name has nothing to do with a serpent or
a serpent bearer, but comes from ancient Arabic patterns that
reflect an Arabic pasture that lies from Hercules to southern Ophiuchus. Here, Rasalhague
(which refers to the serpent bearer) is the shepherd, and Cebalrai
his dog, the name deriving from a phrase that means exactly that.
Physically, Cebalrai is a common orange class K (K2) giant, one of
many relatively cool (4600 Kelvin) "K stars" (headed by brilliant
Arcturus) that populate the naked-eye sky. At a distance of 82
light years, Cebalrai releases 64 solar luminosities into space
from a sphere with a radius 12.5 times that of the Sun. From these data we find that the star has
a very uncertain mass of about twice solar and that it is most
likely now fusing helium into carbon in its deep core, having long
ago given up the hydrogen fusion that powers more common ordinary
stars like the Sun. At first glance, Cebalrai is known more for
its setting as anything else, as it makes a guidepost to the old,
no-longer-recognized constellation
"Poniatowski's Bull," a vee-
shaped figure just to the east of the star, and to an attractive
large star cluster (IC 4665) just to the northeast. Closer
physical examination, however, reveals Cebalrai to be oddly
variable, not in brightness but in size. Doppler measures (which
reveal surface motions through tiny shifts in the wavelengths of
light) show the star to jitter with periods of 0.26, 13.1, and 142
days. The longer period is caused by rotation and dark spots that
swing in and out of view, and is consistent with the rotation
velocity of 2 kilometers per second and an axial tilt. The 13-day
period, however, seems to be a real, though quite subtle,
pulsation, whose origin is not well understood. A similar kind of
pulsation has also been discovered to take place in Arcturus, though with a shorter 8.3 day
period. Sufficiently detailed observations may show many of these
stars to be slightly unstable.